Recently, for multiple-input multiple-output (MIMO) systems, space-time bit-interleaved coded modulation (ST-BICM) using iterative detection has been recognized as a method for achieving near-capacity performance, and thus, enabling the best possible trade between spectral-efficiency and energy-efficiency. However, due to the complexity of the detector (also known as the demapper), this method is not amenable to practical implementation for high-rate MIMO systems targeting spectral-efficiency in excess of 16 coded bps/Hz. This is because the conventional ST-BICM iterative receiver, based on joint or maximum-likelihood (ML) detection, requires the demapper to compute the per-bit a posteriori probability (APP) considering all possible realizations of the simultaneously transmitted symbol streams. Consequently, the demapper complexity associated with the per-bit APP computation is exponential in the product of the number of simultaneously transmitted streams and the bits per symbol. To manage complexity, an approximate demapper using list sphere detection (LSD) has been proposed in B. Hochwald and S. ten Brink, “Achieving near-capacity on a multiple-antenna channel,” IEEE Trans. Commun., vol. 51, no. 3, pp. 389-399, March 2003. Notwithstanding the complexity reduction achieved using this approach, the complexity of LSD-based demappers is still exponential. In D. Garrett, L. Davis, S. ten Brink, B. Hochwald, G. Knagge, “Silicon complexity for maximum likelihood MIMO detection using spherical decoding,” IEEE J. Solid-State Circuits, vol. 39, no. 9, pp. 1544-1552, September 2004, it was shown that the highest rate achievable using sphere-detection based approach, considering the limits of current silicon technology, was about 16 coded bits per second per Hz. Therefore, for high-rate MIMO with near-capacity performance, there remains a need to develop demappers that offer better performance-complexity trades.
To that end, the prior art proposed a parallel single-stream demapper (P-SSD) approach, where streams are demapped independently or in parallel on a per-stream basis rather than jointly as in the conventional joint stream demapper (JSD) approach. The term JSD is used to describe both full-search demappers as well as LSD-based partial-search demappers since both, in the end, perform some sort of joint detection over all of the transmitted streams. For a high-rate 6×4, 16-QAM, MIMO system, using the SSD approach can reduce complexity by at least an order of magnitude relative to its JSD counterpart (implemented based on the LSD approach). For non-overloaded MIMO configurations operating in low-correlation channels, the performance of the SSD method is shown to be comparable to the JSD approach. However, under less ideal conditions (for example, when the channel exhibits significant correlation and/or as fewer receive elements than transmitted streams are used for stream separation), the SSD performs much worse than the JSD. In an attempt to close this gap in these overloaded conditions, the invention proposes a single stream demapper of a successive flavor, where streams are demapped one after another according to some optimal order. The idea is that every stream uses the updated soft information of previously demapped streams to cancel and filter their contributions to the received signal vector. In contrast, the originally developed single stream demappers operate in a parallel fashion, and thus, do not exploit the latest soft information made available as streams are being demapped. Instead, it relies solely on the soft-information from the decoder to reconstruct the interfering symbols. This is a major limitation in these low-complexity receivers as cancellation can never be performed during the first iteration and cancellation during subsequent iterations leaves a greater residual interference than is warranted. Consequently, the receiver is unable to harvest the maximum possible diversity during any iteration.
In the ST-BICM (also known as “iterative” or “turbo” detection) framework, the established theory and practice is to use the per-bit soft-information (or log likelihood ratio value) from the decoder as a priori information for the MIMO detector (or demapper).
It is little surprise then that, in the prior art there are descriptions of various flavors of the parallel single stream demapper, the soft-information from the decoder is precisely what is being prescribed to be used for reconstructing the symbol values which are required for interference cancellation. It is impossible to state with 100% certainty why all of these proposed methods used this same approach (of relying on the decoder for symbol reconstruction and interference cancellation) and did not consider the use of the demapper output to reconstruct symbols (which is exactly what would be required to implement a successive approach). But it is conceivable that there are several reasons for not considering this option:
1) Relying on the demapper output for interference cancellation means that all streams will not benefit from interference cancellation uniformly in the first iteration; the stream that is demapped first does not benefit from any cancellation whereas the stream that is demapped last benefits from the greatest cancellation.
2) It begs the question of an “optimal” order for demapping streams as any sub-optimal ordering can result in worse performance through error propagation.
3) There may be some confusion as to what is to be done in subsequent iterations when a priori information from the decoder becomes available; is soft-information used from the decoder or from the demapper (which becomes available one at a time as streams are being detected)?
4) Last but not least, the idea from the conventional ST-BICM framework that only the decoder can feed a priori information into the demapper, may have inadvertently biased practitioners away from even considering such an option where one demapper output feeds another.
In summary, perhaps the reason that most practitioners seem to have steered away from inventing a similar approach to the present invention is due to some combination of a) bias towards a conventional ST-BICM mode of operation and b) the fact that there are one too many side issues to be considered and resolved in order to implement the successive demapping option.
Compared to MIMO detectors based on the parallel approach, the present solution is able to perform interference cancellation from the very first iteration as well as exploit the newer soft-information from demappers of previously detected streams. Consequently, the successive approach used in the present invention a) provides additional diversity gain (or increase battery life or reduce transmit power), b) decodes packets with fewer iterations (incurs less latency), c) works with smaller receive arrays, d) can work in less “MIMO friendly” channels.
The present invention provides that:
1) For a 4×4, 16-QAM, MIMO system, the performance gain (relative to the parallel approach) for a single iteration is about 0.75 dB.
2) For a 4×3, 16-QAM, MIMO system, the performance gain (relative to the parallel approach) for a single iteration is infinite. By “infinite”, it is meant that a satisfactory packet error rate can never be achieved using the parallel approach, no matter how high the transmit power.
In the present invention, there is a successive-single stream demapper (S-SSD) and their performance is compared to that of the previously developed parallel-single stream demapper (P-SSD).